1. Field of the Invention
This invention relates to an optical information recording medium, and in particular a recording medium wherein an organic polymeric material is used as a recording layer and optical information can be repeatedly recorded, reproduced and erased.
2. Description of the Related Art
In recent years, due to the progress of laser technology, much research is being carried out on optical information recording media wherein information is written, read and erased by means of a laser beam. There are moreover two methods of recording information on such media, namely a photon mode wherein the laser light is used as photons, and a heat mode wherein the laser light is used as heat.
Photon mode recording media are characterized by high sensitivity, and by the high speed with which information can be recorded. Examples of such recording media have been proposed wherein a photochromic material is used as the recording layer, for example spiropyran (Published Unexamined Japanese Patent Application (PUJPA) No. 60-177084 and Preprints of the 50th Annual Spring Meeting of the Japan Chemical Society No. 1, p. 253 (1985)), fulgide or indigo (PUJPA) No. 61-128244 and J. Chem. Soc., Perkin Trans. 1, 197 (1981) and 2, 341 (1981)), and diallylethene (PUJPA No. 63-24245). However, although the recording media described in these proposals have the above advantages, the recording layer tends to undergo a back photochromic reaction and is less resistance to the reading light. As a result, their stability in the colored state and reproduction repeatability are not good, and their information retention properties are also poor.
In heat mode optical recording media, on the other hand, when light irradiates the recording layer, it causes local melting, thermal deformation or phase transition at the irradiated portions. Differences in optical properties are therefore produced at the portions in the media where the changes occur, and this effects the recording of the information. The reproduction of the recording is effected by detecting these differences of optical properties (reflectance and refractive index, etc.). Examples of recording media of this type use magneto-optical materials such as TbFe, GdFe and TbFeCo as the recording layer, or chalcogen alloys such as TeOx, Te-Ge, Te-Ge-Sb and Te-Ge-Sb-Tl as the recording layer as according to Ovshinsky et al. Recording media having these inorganic materials however have disadvantages of low sensitivity, toxicity and poor retention of recorded information. Further, high speed recording and erasure is impossible, and overwrite performance is inadequate.
To overcome the disadvantages of these inorganic materials, intensive research has begun to be carried out on recording media using organic polymeric materials, for example using a mixture of thermoplastic polymers and pigments which absorb in the near infrared as the recording layer 58-48245), or using the phase separation of polymer blends for recording. Further, recording media using polymeric liquid crystals are also known (see Polym. Commun. 24, 364, 1983; PUJPA Nos. 59-10930 and 60-114823; Macromolecules, 17, 1004-1009, 1984; Advances in Polymers, 59, 37, 1984; PUJPA Nos. 61-28004 and 62-175939; DE 3500838 Al; and PUJPA Nos. 58-125247 and 63-98852). Recording media using these organic polymeric materials were, however, unsatisfactory from the viewpoints of recording sensitivity, reproduction repeatability, recording speed, contrast and/or recording retention.
Recently, polydiacetylenes with conjugated .pi. electron have been reported to undergo phase transitions by the application of external energy such as light energy, heat energy, mechanical energy or electrical energy (J. Chem. Phy. Vol. 70(9), No. 1, 4387 et seq. (1979), Vol. 71(1), 206 (1979), and Vol. 67, No. 8, 3616 et seq. (1977); Polymer Preprints, Japan, Vol. 35, No. 6, 393 (1986); Solid State Physics, Vol. 20, No. 11, 845 et seq. (1985); Japan Physics Soc., 84th Autumn Meeting, 323; and Japan Physics Soc. 85th Autumn Meeting, 323). Laser recording media using polydiacetylenes are disclosed in PUJPA No. 56-147807, while recording media using pigments in conjunction with polydiacetylenes are disclosed in PUJPA Nos. 60-192686 and 62-175940. The media disclosed in these references are, however, all based on an irreversible change of color tone of polydiacetylenes, and although information can be recorded and reproduced, it cannot be erased. Further, recording media based on a change in the main chain structure of the polydiacetylene between an acetylene configuration and a butatriene configuration, are disclosed in PUJPA Nos. 61-203448 and 61-203454. These media however are unsatisfactory from the viewpoint of contrast, recording retention, erasability and repeatability.